The present invention relates to a fuel injection pump for a diesel engine, and more particularly relates to a type of diesel fuel injection pump in which the injection of fuel is performed in two phases: an earlier pilot fuel injection spirt and a later main fuel injection spirt.
In a diesel engine, the diesel fuel is injected at high pressure by a diesel fuel injection pump through fuel injectors into the cylinders of the engine in turn upon their compression strokes, and ignites due to the natural compression in the cylinders and is combusted therein without any special electrical or mechanical ignition means being required. Diesel knocking, especially under low load, can present a problem, and therefore there has been proposed a per se known type of fuel injection control, in which into each cylinder of the engine, on its compression stroke, there are injected two successive fuel injection spirts: a pilot fuel injection spirt, of a relatively low volume of diesel fuel, which is injected at an earlier time in order to initiate the combustion process within the cylinder, and a main fuel injection spirt, of a relatively large volume of diesel fuel, which is injected at a slightly later time in the compression stroke in order to actually provide a proper amount of fuel for engine operation. The combustion of this later main fuel injection spirt is much aided by the fact that the low volume pilot fuel injection spirt is already being combusted within the combustion chamber, and by this concept diesel knocking, especially at low engine load condition such as the idling condition, can be effectively eliminated, because of the retardation of the overall combustion process which causes smooth and gradual combustion in the combustion chamber to be achieved.
There is known a type of fuel injection pump for a diesel internal combustion engine which includes a plunger which reciprocates to and fro in a bore defined in a housing, a high pressure chamber being defined between one end of the plunger and the end of the bore. During the suction stroke of the plunger as this high pressure chamber expands in size, diesel fuel is sucked into this high pressure chamber from a quantity of diesel fuel contained in a relatively low pressure chamber through a fuel supply passage; and during the compression stroke of the plunger as the high pressure chamber subsequently contracts in size, this diesel fuel in the high pressure chamber is squeezed and is brought to a high pressure and is ejected through an injection passage therefor to a fuel injector in a cylinder of the diesel internal combustion engine. Sometimes, in the case that the diesel fuel injection pump is a so called distribution type pump, the plunger is rotated as it reciprocates by an input shaft which is rotationally coupled to it although not axially coupled to it, and by a per se well known construction the spirt of highly compressed diesel fuel is directed to the appropriate one of the plurality of cylinders of the internal combustion engine. Now, such a fuel injection pump injects an amount of diesel fuel in each pump stroke which is regulated by a fuel injection amount control means which selectively vents the high pressure chamber. This control means ceases to vent the high pressure chamber when it is appropriate to start the fuel injection spirt, during the compression stroke of the plunger, and at this instant the almost incompressible diesel fuel in the high pressure chamber starts to be squeezed and injected, as explained above. When it is appropriate to terminate the fuel injection spirt, then the control means starts again to vent the high pressure chamber, and at this instant the diesel fuel in the high pressure chamber ceases to be squeezed and therefore the injection is immediately stopped.
In the case of a mechanical diesel fuel injection pump, it has been conventional for this high pressure chamber selective venting means to be a spill ring, which is mechanically positioned according to the position of the accelerator pedal which is controlling the load on the engine, and whose position controls the timing instant of the end of the non-vented time period of the high pressure chamber. In such a mechanical type of fuel injection diesel pump, it is very difficult to perform such a two phase type of fuel injection as described above in which a pilot fuel injection spirt of low volume precedes the main fuel injection spirt, because of the fact that typically the accelerator pedal simply positions the spill ring through a simple linkage, and in such a construction there is no good way of implementing the two phase fuel injection method described above in a sufficiently flexible manner.
However, nowadays electronically controlled fuel injection pumps are coming into use, in which the selective venting of the high pressure chamber is performed, not mechanically by the use of a spill ring, but electronically by an electromagnetic valve which is controlled by an electronic control system such as one incorporating a microcomputer. In such an electronic fuel injection pump, the electronic control system, for each spirt of fuel injection, calculates how much fuel is to be injected in this spirt, and then at an appropriate time point for the start of fuel injection closes said electromagnetic valve, so as to terminate fuel spilling from the high pressure chamber and so as thereby to start fuel injection. After the electronic control system has calculated that the proper amount of fuel has been injected by the movement of the plunger in the direction to reduce the size of the high pressure chamber, then said control system opens said electromagnetic valve for fuel spilling again, thus immediately terminating fuel injection. In such an electronic type of fuel injection pump, in theory the two phase fuel injection method described above could be well implemented.
However, the type of electromagnetic valve for fuel spilling that has heretofore been proposed for such an electronic type of fuel injection pump has been of the prior art direct action type shown in FIG. 1 of the accompanying drawings as an example. In the fuel injection pump of this figure, of which only a right side part including the electromagnetic valve 3 for fuel spilling is shown in longitudinal cross section, the plunger described above is designated by the reference numeral 5, and reciprocates as it rotates, and the high pressure chamber 7 for pumping the fuel is defined at the right hand end of this plunger 5, while diesel fuel which is squeezed in this high pressure chamber 7 by rightward movement of the plunger 5, when the high pressure chamber 7 is not being vented by the valve 3, is squirted towards a fuel injector of the diesel engine via a delivery valve 9. The electromagnetic valve 3 comprises a solenoid coil 15 and a valve element 11 which slides to and fro in a bore, being biased leftward in the figure by a compression coil spring 13 and being attracted rightward in the figure by the magnetic action of the solenoid coil 15, when actuating electrical energy is supplied to the valve 3. The tip 21 of the valve element 13 is opposed to a hole in a valve seat 23 which opens between the high pressure chamber 7 and a drain passage 25, and thus, when the solenoid coil 15 is not supplied with actuating electrical energy, the valve element 11 is biased leftwards in the figure by the compression coil spring 13 so that its tip presses against the hole in the valve seat 23 and closes it, preventing the high pressure chamber 7 from being drained and thus causing fuel injection through the delivery valve 9 to be performed, when the plunger 5 moves rightwards on its compression stroke. But, on the other hand, when the solenoid coil 15 is supplied with actuating electrical energy, the valve element 11 is attracted rightwards in the figure against the biasing action of the compression coil spring 13 which is overcome so that its tip is moved away from the hole in the valve seat 23 and opens this hole, thus opening the high pressure chamber 7 to the drain 25 and thus causing fuel injection through the delivery valve 9 to be prevented by draining and relieving the pressure inside said chamber 7, even when the plunger 5 moves rightwards on its compression stroke.
In such a type of direct action electromagnetic valve for fuel spilling, when it is attempted to be applied to fuel injection of the above described sort performed in two phases, an earlier pilot fuel injection spirt and a later main fuel injection spirt, the problem arises that its responsiveness is too slow. For operation in such a mode, very high speed response is needed in order for the fuel injection pump to be able to inject the pilot fuel injection spirt, since this spirt is of relatively small volume and has a relatively short duration, and the response time required of the electromagnetic valve is typically of the order of one to two milliseconds or so. Such a direct action electromagnetic valve for fuel spilling as described above is not capable of providing such a good response time, and accordingly as yet no diesel fuel injection pump which injects a pilot injection fuel spirt prior to the main fuel injection fuel spirt has been put into practical use.
Further, in this type of direct action electromagnetic valve for fuel spilling, since the pressure receiving area of the valve element 11, i.e. the opening area of the hole through the valve seat 23, is quite large, a large force is applied to it by the very high pressure generated in the high pressure chamber 7 on the compression stroke of the plunger 5. Therefore, in order to keep the hole through the valve seat 23 closed when fuel injection is being required, a large force is required for pressing the valve element 11 against it. In the case of an electromagnetic valve for fuel spilling of a normally closed type, this means that the spring 13 which biases the valve element 11 is required to be large in size and heavy, and correspondingly the solenoid coil 15 is required to be large and powerful in order to counteract the biasing force of this spring 13, and therefore the power consumption of this solenoid coil becomes large, and the drive circuitry therefore becomes massive and complicated. Even in the case of an electromagnetic valve for fuel spilling of a normally open type, although the biasing spring is not required to be extremely massive, still this problem of the solenoid coil and its circuitry being required to be powerful and bulky remains.
Another type of problem that can occur with an electromagnetic valve for fuel spilling, especially in the case that such a valve is of the normally open type which is closed by supply of electric current to its solenoid coil, is that upon cessation of the supply of electric current to said solenoid coil the valve does not respond immediately. This can be due to the effect of residual magnetism, and can cause irregularities and inaccuracies in the supply of injected diesel fuel, when such an electromagnetic valve is applied for fuel spilling. In the case of a normally open type electromagnetic valve, this can cause the risk that fuel injection should linger on for some time after it should have stopped, and this can cause the response of the valve to be degraded. On the other hand, in the case of a normally closed type electromagnetic valve, this can cause the risk that fuel injection is not properly started some time after it should have started, and again this can cause the response and the performance of the valve to be degraded. Particularly in the case outlined above, wherein the fuel injection is intended to be performed in two phases, a pilot fuel injection spirt and then subsequently a main fuel injection spirt, this poor response time of the valve can be very troublesome.
It might be conceived of for the control system for the electromagnetic valve to make an allowance for this type of residual magnetism effect, but the problem arises that, because of the unpredictable nature of residual magnetism effects, such an effect is impossible to predict accurately, and accordingly this solution is not really workable.